1. Field of the Invention
The present invention relates to a vibration control apparatus and method for controlling generation of a vibration wave in an electrostrictive element.
2. Description of the Related Art
Conventionally, there have been known vibration wave actuators such as ultrasonic wave motors. A vibration wave actuator of this kind includes an electrostrictive element, such as a piezoelectric element, which is adapted to generate mechanical vibration when applied with an alternating voltage. The resultant vibration energy is utilized for production of a driving force.
Other vibration wave actuators have also been realized, which are adapted to excite a higher-order vibration mode (see, Japanese Laid-open Patent Publications Nos. 6-327271 and 2003-333391). To improve the startability, the vibration wave actuator disclosed in Japanese Laid-open Patent Publication No. 6-327271 generates, at the time of startup, vibration different from vibration of a primary drive mode.
In order to remove dust adhered around an image pickup element, the actuator disclosed in Japanese Laid-open Patent Publication No. 2003-333391 includes a piezoelectric element, which is disposed on an optical element located at a front surface of the image pickup element and adapted to vibrate with at least two resonance frequencies in sequence.
In the actuators disclosed in Japanese Laid-open Patent Publications Nos. 6-327271 and 2003-333391, however, the electrostrictive element is only provided with electrodes designed in correspondence to a mode shape of a predetermined order vibration mode, but not provided with electrodes corresponding to mode shapes of higher-order vibration modes. For this reason, the vibration efficiency of the actuators is lowered when a vibration mode other than the predetermined mode is excited using the electrodes formed in the electrostrictive element.
On the other hand, in order to provide an electrostrictive element with electrodes corresponding to mode shapes of higher-order vibration modes, a plurality of electrostrictive element parts each formed with electrodes in correspondence to a mode shape concerned must be stacked into one piece, for example, resulting in decrease in the utilization efficiency of the electrostrictive element.
In the actuators disclosed in Japanese Laid-open Patent Publications Nos. 6-327271 and 2003-333391, the electrodes are provided solely at positions of loops in the mode shape of a predetermined order vibration mode, making it impossible to directly excite vibrations at positions of loops in a mode shape of another order vibration mode.